VLF
(very low frequency) ground canceling metal detectors work
on the principle of conductivity. These metal detectors will
locate any metal with a conductive surface within 45-60 cm
(18" to two feet) of the ground surface. A signal is heard
because an electromagnetic field is generated and caused to
flow out into space away from the search coil. As the lines
of the electromagnetic field penetrate the surface of the
metal, tiny circulating electrical currents called eddy currents
are caused to flow in the surface of the metal. These eddy
currents then generate their own electromagnetic fields which
radiate in all directions in space. Those currents which radiate
directly upwards toward the receiver's search coil windings
are picked up. A reduced voltage is then fed into the amplifying
wires where it is conditioned to drive a loudspeaker, meter
or headphones. The larger the surface, the greater the signal.
Metal detectors do not react to mass but to surface area.
In other words, a flat coin will make a larger signal than
a coin on its edge.
Most VLF metal detectors are equipped with an alternative
mode called a TR discriminator mode. This mode is 90% accurate
in being able to discriminate between two classes of non-ferrous
and ferrous (iron containing) substances:
- gold,
silver, copper, lead, zinc and nickel. These metals have
better conductivity than do those in the second group
- iron,
tin, foil-coated wrappers and bottle tops
Pin Pointing the Target
When a signal is heard, and
do not ignore the faint ones, immediately swing back over
the area to pinpoint it. An accurately pinpointed target will
save you hours of digging work.
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